@djqsrv as you are in your 40s like me and getting yourself back, I found that to start 3-4 short (90 minute max) sessions worked well. For the first few months I gave myself an on/off schedule so no two days in a row to let myself recover .... I have dropped a lot of weight in the last year, 35lbs before beginning tennis, another 20 after .... that is at least what worked for me ... everyone is different.
@zill since you are so young and have good conditioning, I think you can do what you prefer time wise. for me now, I like my 5-6 days a week of 1.5 - 2 hours each schedule. I think that long sessions on those days when things aren't that fantastic might be a bit frustrating.
In terms of timing, hit as many balls as you can, take a few clinics if your club has them, grab a ball machine if you like them. The more repetitions you get, the more that timing will come back. It definitely came back for me after a much longer break, but I didn't just jump into matches or playing sets ... I practiced first to get my "groove" back!
@TenFanLA I would love to have some of the aspects of my 20s back ... that fluidity is one of them! Ain't gonna happen. And I think you are right, we replace it with being way smarter!